I think the best we have is what SteamBot says: the mean/median profile levels are 9 and 7, respectively, based on data sourced from approximately 3.9m accounts.
Hmmm. Then I agree with you on the fact that keeping foil drops restricted to level 10+ users would have make quite a bit of a difference!
My best one was a foil from a game almost nobody bought.
In the end, it sold at 5$ but I had it up for sale for about half a year =/
I probably made more than that 5$ with all the low value cards I sold in 6 months.
I think that I made around 7€ once. but that's it. A friend of mine sold one for 13€. Which is -unthinkable- nowadays.